To automatically populate entity from form request, you have to bind your entity to the FormType class. You might be missing this bind from your UserType class. In form UserType you should add a method configureOptions inside it you specify the entity class name in data_class key. To have more insights into it see Symfony Forms and scroll down to Setting the data_class topic. Also See example below where I have listed the usage.

Now, when you use the form, your entity is resolved from the request as shown below $site = $form->getData() will return Site entity. This is quite tricky to get right at the first time. You have to practice it a lot until you get hang of it. Let us know if you have confusions following it.